This application relates to an improved method for the production of polycarbonates.
Polycarbonates are conventionally produced by the reaction of a dihydric phenol, such as bisphenol A (BPA), and a diarylcarbonate, such as diphenyl carbonate (DPC). BPA and DPC are combined together is a monomer mixing tank, and the resulting mixture is fed into the reactor to produce the polycarbonate. The characteristics of the product polycarbonate is determined in part by the relative amounts of BPA and DPC in the monomer mix tank. This arises from a relationship between the end-cap level (i.e., the termination of a polymer in a manner unsuitable for further chain extension) and monomer ratio.
This relationship between product characteristics and monomer ratio makes it desirable to test the levels of BPA and DPC in the mix tank. At present, this is done using an HPLC analysis. The efficacy of the HPLC analysis is limited, however, because in some instances, catalyst is added to form an equilibrated oligomer-containing mixture in the mix tank. When HPLC is used to analyze such mixtures, the measured ratio determined by HPLC provides an inaccurate measure of the actual ratio of BPA to DPC. Thus, when an equilibrated mixture is formed, HPLC fails to accurately predict the characteristics of the final product, and cannot provide a reliable guide to adjustment of the amounts of materials added to the monomer mix tank. Some alternative form of monitoring is therefore required for use in such systems.
The present invention provides a new method for evaluating the stoichiometric ratio of dihydric phenol to diarylcarbonate during production of polycarbonates which does not suffer from the deficiencies of the HPLC technique. In accordance with this method, infrared analysis, preferably using an FTIR spectrometer, is utilized to detect the amount of BPA or other dihydric phenol in the mix tank. In one embodiment of the invention, one or both of two characteristic peaks, reflecting the methyl CH bonds and hydroxyl OH bonds are evaluated. These peaks occur equally in BPA monomers and oligomers, and hence are independent of the degree of oligomerization or polymerization which has occurred. For this reason, monitoring of stoichiometry can be performed on the contents of the mix tank, or on the products from or contents of any downstream reactors. Using a calibration curve, the absorbance associated with these bonds is converted into a measure of the concentration of BPA and from that to first moles and then grams of BPA in the mixture being tested. The amount of DPC is determined by subtracting the number of grams of BPA from the total initial sample weight. In another embodiment of the invention, a ratio of the size of the peaks associated with DPC carbonyl and BPA methyl is used directly to determine the ratio of DPC to BPA.